Why Pregnant Women Don't Tip Over

Dec. 12, 2007 -- Pregnant women can thank their aching backs for keeping
them upright, a new study shows.

The study, published tomorrow in Nature, explains that women's spines
are built differently from men's spines.

The study shows that the lower part of a woman's spine is built to curve
more during pregnancy. That adjustment helps women hold their center
of gravity while pregnancy pushes their waistline way beyond their hips.

"The body must change in dramatic ways to accommodate the baby, and
these changes affect a woman's stability and posture. It turns out that
enhanced curvature and reinforcement of the lower spine are key to maintaining
normal activities during pregnancy," says Whitcome, who is a postdoctoral
researcher at Harvard University's anthropology department.

Eons of Pregnancy Back Pain

Whitcome and colleagues studied 19 pregnant women, analyzing how the women
stood and walked at various points during pregnancy.

As the pregnancies progressed, the women had more curvature of their lower
spine.

This has happened for at least 2 million years, judging by fossils of the
earliest hominids that walked on two feet, Whitcome's team notes.

Curving the spine during pregnancy probably never felt good.

"It is reasonable to hypothesize that fatigue and pain in the lower back
muscle affected early hominid mothers just as they do modern mothers,"
write the researchers.

But today's pregnant women are better off in at least two ways, according to
the study.

First, a modern pregnant woman isn't likely to be preyed upon by a wild
animal when her back pain gets to her. Second, she's got fewer vertebrae than
her ancient ancestors, making it easier to carry things, walk, and run, if need
be.

"Any mother can attest to the awkwardness of standing and walking while
balancing pregnancy weight in front of the body," Liza Shapiro, PhD, says
in a news release.

"Yet our research shows their spines have evolved to make pregnancy
safer and less painful than it might have been if these adaptations had not
occurred," says Shapiro, an associate professor of anthropology the
University of Texas at Austin.

Whitcome and Shapiro worked on the study with Daniel Lieberman, PhD,
professor of biological anthropology at Harvard University.